fix: remove banned V-language v-adapter (route via zig-unified-api-adapter) (#65)

## Summary

V-language is banned estate-wide; the sanctioned connector layer is
`zig-unified-api-adapter`. This PR removes the V-language adapter from
echidna — the worst single V-lang footprint in the estate (9 live source
files).

## Changes

Removed `src/interfaces/v-adapter/` (9 V source files, all `module
main`):
`boj.v`, `graphql.v`, `grpc.v`, `overlay.v`, `overlay_graphql.v`,
`overlay_grpc.v`, `rest.v`, `tentacles.v`, `typell.v`.

## Why no wiring changes were needed

The directory was completely unwired:

- **Not a Cargo workspace member** — `Cargo.toml` lists only
`src/interfaces/{graphql,grpc,rest}` (plus the `crates/*`), never
`v-adapter`.
- **No `mod` declaration** referencing it anywhere.
- **No tracked build file, `Justfile`, `build.rs`, or CI workflow**
references `v-adapter`/`v_adapter`.
- `src/interfaces/README.md` documents only graphql/grpc/rest and
contains no v-adapter reference.

`git grep` for `v-adapter`/`v_adapter` across tracked non-data files
returns **no remaining references** after removal. No dangling paths, no
`// TODO` stubs required.

## Validation

- Confirmed the 9 files are V source (`module main`, V syntax), not Coq.
- Post-removal dangling-reference scan: CLEAN.
- `cargo build --offline` fails on a pre-existing `schemars`
dependency-resolution issue in the unrelated `echidna-mcp` crate
(offline cache miss, not a regression — `v-adapter` was never a Cargo
member so its removal cannot affect the build graph).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: hyperpolymath <hyperpolymath@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath